Self-assembled monolayers (SAMs) can form by the spontaneous adsorption of functionalizing molecules from a solution or a gas onto a substrate (for example, a metal or a metal oxide) in a single layer. SAM-forming molecules generally comprise a headgroup, which can interact with the substrate, while the remaining portion of the molecule can acquire some order from its interaction with neighboring molecules in the monolayer. For example, the molecules can be attached by a chemical bond to the substrate surface and can adopt a preferred orientation with respect to that surface and even with respect to each other.
SAMs can form on a variety of types of substrates (including those comprising a coating on a physical support) depending upon the specific chemical and/or physical nature of the headgroup. For example, alkylthiols and disulfides can form SAMs on gold, silver, palladium, and copper, and silanes can form SAMs on silicon oxide. The preparation, characterization, and utilization of SAMs has been an important field of research because of the ability of SAMs to change and control the properties (for example, the wetting, lubrication, and/or corrosion properties) of substrate surfaces.
Various techniques have been used for patterning self-assembled monolayers on substrate surfaces. Many of these techniques have involved first covering the substrate entirely with a SAM and then removing the SAM in some areas by using, for example, ultraviolet light, an electron beam, bombarded atoms, or the probe of a scanning probe microscope (scanning tunneling microscope or atomic force microscope). Another patterning technique called microcontact printing (μCP) has been used to form a SAM on desired areas of a substrate surface during a printing step (for example, with printed feature sizes of less than one micrometer being achievable). The patterned SAMs resulting from such patterning techniques have then served as resists for selectively etching the substrates.
Microcontact printing SAMs generally involves applying an ink composition comprising functionalizing molecules to a relief-patterned elastomeric stamp (for example, a poly(dimethylsiloxane) (PDMS) stamp) and then contacting the inked stamp to a substrate surface, usually a metal (for example, gold, silver, palladium, or copper) or metal oxide (for example, indium-tin oxide) surface, so that SAMs form in the regions of contact between the stamp and the substrate. It has been relatively simple to use microcontact printing to change the surface properties (for example, the wetting characteristics) of a substrate, and, although it has been possible to employ microcontact printing for the chemical etch-patterning of substrates (for example, relatively thin gold films) with high contrast and resolution, such methods have generally required the use of chemical etchants with certain undesirable characteristics that have limited commercial implementation. Specifically, heretofore it has not been possible to etch pattern metal substrates (including metal-coated physical supports) using microcontact printed SAM mask patterns in combination with chemical etchants having desirably long lifetimes and desirably high etch rates and etch capacities.
Even high quality SAMs having relatively few defects can be ineffective in providing etch protection, if the molecules forming the SAMs lack resistance to the chemical etchants that are utilized. Potassium iodide/iodine-based etchants (KI/I2; a “tri-iodide” etchant) are commercially available and offer a compelling combination of stability (for example, for periods of weeks), speed (for example, etch rates of about 25-660 nanometers per minute), capacity (for example, greater than about 20 grams of metal dissolved per liter of etchant solution), and safety. The SAM-forming molecules typically used in etch-patterning, however, generally do not exhibit sufficient resistance to tri-iodide etchants to enable effective patterning. Specifically, for the SAM-forming molecules used with such etchants to date, metal underlying a patterned SAM generally has been etched essentially as rapidly as metal not covered by the SAM.
Conventional SAMs have shown resistance to a variety of other chemical etchants that have been used for etching gold (including cyanide/oxygen-, ferrocyanide/ferricyanide-, and thiourea-based etchant systems), but these etchants have their own deficiencies. Cyanide/oxygen-based systems are generally relatively slow (for example, etch rates of about 2-3 nanometers per minute) and can present toxicity issues. Ferrocyanide/ferricyanide mixtures can be less toxic than etchants based upon free cyanide but also are relatively slow (for example, etch rates of about 2-4 nanometers per minute) and of relatively low capacity. Thiourea-based etchants with ferric ions as oxidizing species are relatively stable (for example, several hours of stable activity) but relatively slow (for example, etch rates of about 10 nanometers per minute for gold). Thiourea-based etchants with hydrogen peroxide as oxidizing species are relatively faster (for example, etch rates of about 100 nanometers per minute for gold) but are relatively unstable (for example, stable for periods of only minutes to hours).